LOS ANGELES — Wall Street is not thrilled with the new fall television season.

On Thursday, two prominent media analysts issued reports expressing concern about the new season. So far, few new shows have taken off and ratings have tumbled at ABC, CBS and Fox in the adults 18-49 demographic that advertisers covet. Both CBS and Fox are off by more than 20 percent while ABC is down 13 percent.

NBC, which has struggled for years, is the only network to improve in that key demographic, thanks to its move of “The Voice” to the fall, the early success of the drama “Revolution” and new comedies “Go On” and “The New Normal.” Overall, NBC is up 12 percent in 18-49 viewers compared with last season.

“There is little doubt that early 2012-13 network results have been disappointing,” wrote Nomura Equity Research analyst Michael Nathanson. While Nathanson notes that it is usually best to judge a network’s performance after ratings information that includes digital video recorders is factored in, he thinks those numbers won’t entirely make up for the dramatic drop the networks are experiencing.

Media analyst Anthony DiClemente, with Barclays Equity Research, says the new shows are the problem. “We believe the weakness in the broadcast ratings is primarily a content-driven issue; most of the freshman shows have been a disappointment.”

So far, the only new show to be canceled is CBS’ Friday night legal drama “Made in Jersey.” But other offerings that have failed to find an audience include Fox’s “Mob Doctor,” NBC’s “Animal Practice” and “Guys with Kids,” and CBS’ “Partners.”

Another factor hurting the networks is the NFL. Not only are ESPN’s “Monday Night Football” ratings up slightly this season, the NFL Network is now carrying Thursday games all season.

“We believe the uptick in viewership, particularly on Thursday night, is having a cannibalistic effect on original program live ratings, as football now competes with originals three nights a week (Sunday, Monday, Thursday),” DiClemente said.

To be sure, the growing popularity of DVRs is also changing the way people consume television. Sometimes a show’s audience grows by as much as 50 percent when recorded viewing is factored in.